The present invention relates to a process for preparing styrene oxide by the reaction of styrene and hydrogen peroxide in the presence of a catalyst.
The application field of styrene oxide covers a wide range; for example, styrene oxide is used as a stabilizer for high polymers, an ultraviolet ray absorber, a starting material of drugs, a stabilizer for solvents, or as a starting material of phenethyl alcohol and phenylaldehyde which are useful as a synthetic perfume and a sweetening material.
For the preparation of styrene oxide, the method of epoxidizing styrene using an organic peracid is most common as is described in Japanese Patent Laid-Open No. 149,271/1980. But this method involves the following drawbacks and is not always satisfactory.
(a) There occurs an addition reaction of a radical to styrene which radical is formed by radical decomposition of an organic peracid during the reaction of oxidizing styrene with the organic peracid, resulting in lowered selectivity of styrene oxide relative to styrene.
(b) An organic acid byproduced after the reaction causes cleavage of the styrene oxide produced, to form an ester and a hydroxy compound, resulting in that the selectivity of styrene oxide relative to styrene is lowered.
(c) Peracetic acid which is most easily available industrially among organic peracids is prepared by a so-called Daicel-Wacker process involving air oxidation of acetaldehyde. But it is a very expensive oxidizing agent.
(d) Close attention must be paid to operation and apparatus in order to avoid the risk involved in the use of an organic peracid.
On the other hand, in the oxidation reaction using hydrogen peroxide, the by-product is only water and there arises no problem related to evironmental pollution; besides, hydrogen peroxide is easily available industrially and inexpensive, so in principle hydrogen peroxide is a desirable epoxidizing agent. However, in the reaction of styrene and hydrogen peroxide to form an epoxide, both the conversion of styrene and the selectivity to epoxide are low. The reason why the conversion is low is that in a low-temperature reaction hydrogen peroxide remains unreacted, while in a high-temperature reaction it decomposes to produce oxygen, and thus hydrogen peroxide is not effectively consumed in the reaction.
The reason why selectivity to epoxide is low is that a polyol is formed by the water which is introduced into the reaction system together with hydrogen peroxide and also by the water which is produced by the reaction.
The reactivity in epoxidation of styrene is as shown in the following table [see "Encyclopedia of Polymer Science and Technology," Vol. VI, Interscience Publishers, N.Y. (1967), p. 83]. It is seen from this table that the relative reactivity in epoxidation of styrene is slower than that of other olefins, for example, about one-tenth of the relative reactivity in epoxidation of cyclohexene, thus indicating a very slot epoxidation reaction of styrene.
______________________________________ Olefin Relative Reactivity ______________________________________ CH.sub.2 .dbd.CH.sub.2 1 C.sub.6 H.sub.5 CH.sub.2 --CH.dbd.CH.sub.2 11 R--CH.dbd.CH.sub.2 25 Ar--CH.dbd.CH--Ar 27 Ar--CH.dbd.CH.sub.2 60 Ar--CH.dbd.CH--R 240 (Ar).sub.2 C.dbd.CH.sub.2 250 R--CH.dbd.CH--R 500 (R).sub.2 C.dbd.CH.sub.2 500 cyclohexene 675 cycloheptene 900 cyclopentene 1000 (R).sub.2 C.dbd.CH--R 6500 (R).sub.2 C.dbd.C(R).sub.2 &gt;&gt;6500 ______________________________________
In the above table, Ar and R represent aryl and alkyl, respectively.
Heretofore, in the preparation of styrene oxide by the reaction of styrene and hydrogen peroxide, there has been proposed the use of a specific catalyst for solving the above-mentioned problems. For example, C. Bentoureluro et al. (J. Org. Chem., 53, 1553, 1988) reports that styrene oxide is obtained in 74% yield (based on hydrogen peroxide) by using a quaternary ammonium salt of phosphotungstic acid as a hydrogen peroxide epoxidizing catalyst. Although this method affords a greatly improved yield as compared with other conventional methods, it is difficult to adopt this method industrially because the quaternary ammonium salt (interphase transfer catalyst) used as a catalyst component is very expensive.
In Japanese Patent Laid-Open No. 129,276/1980 there is proposed a method of reacting styrene and hydrogen peroxide in the presence of arsenic oxide and a 3,5-di-tert-butyl-4-hydroxytoluene. However, this method is disadvantageous in that when arsenic oxide is used together with aqueous hydrogen peroxide, the hydrogen peroxide will decompose rapidly, or the epoxidizing speed is uneconomical. Further, since arsenic compounds are highly toxic, it is necessary to exercise ample care about the manufacturing equipment in order to prevent the workers or users from being poisoned during manufacture or at the time of use due to incorporation of an arsenic compound in the product.
U.S. Pat. No. 3,806,467 proposes a process for preparing an epoxide by the reaction of an olefin and hydrogen peroxide in the presence of a bis(tri-n-methyltinoxy)molybdic acid catalyst. According to the working examples thereof, the yield of cyclohexene epoxide is high and the process is an effective process, but the yield of styrene oxide is a little less than 3% (based on hydrogen peroxide) and thus it cannot be said that the process is a preferable process for the preparation of styrene oxide. The reason for such a poor yield of styrene oxide is presumed to be because of oxidative cleavage of the styrene oxide produced, resulting in by-production of benzaldehyde and benzoic acid.